An image obtained by capturing an image displayed on a monitor screen with a camera is grainy due to a dot pattern of the screen appearing in the captured image. In order to eliminate the graininess from such an image, it is conceivable to use a well-known noise removal technique to smooth the image. Smoothing processing is processing for replacing the pixel value of each pixel with an average of the pixel values of pixels in a neighboring region of the pixel. Here, simple averaging processing may be performed, or weighted averaging of the pixel values of the neighboring region using various types of filters such as Gaussian filters or median filters may also be performed. The size (often expressed by “radius”) of the neighboring region serves as a parameter of the smoothing processing. The larger the size of the neighboring region, the easier the noise will be removed.
However, if a noisy image is smoothed, not only noise will be removed but also outlines (edges) will be blurred. In order to address such problems, Patent Literature 1 discloses a technique in which the size of a neighboring region of each target pixel is set so that the strength of edges in the neighboring region is at a threshold or greater, and the smoothing strength is changed according to the size of the neighboring region. In other words, the size of the neighboring region is set to be smaller in a section in which edges are converged, whereas the size of the neighboring region is set to be larger in a section in which less edges are present.